190 



COLLEGE ZOOLOGY 



verse fission. Nais is light brown in color, 

 2 to 4 mm. long, and consists of from 15 to 

 37 segments. It lives among algae and may 

 reproduce by budding. Tubifex tubifex is 

 reddish in color and about 4 cm. long. It 

 lives in a tube from which the posterior end 

 projects and waves back and forth. Often 

 large numbers occur in patches on muddy 

 bottoms. 



Among the smallest of oligochaetes are 

 species of Chaetogaster that may be only 

 0.44 mm. long. The largest ones are known 

 from South Australia, where Megascolides 

 austrdis may attain a length of 7 feet. The 

 number of segments in oligochaetes varies 

 from 7 in Aeolosoma to over 600 in Rhino- 

 drilus. 



Leeches 



The class Hirudinea contains annelids 

 that are usually flattened dorsoventrally, but 

 differ externally from the flatworms in being 

 distinctly segmented. They differ from other 

 annelids in the lack of setae (except in one 

 genus), and in the presence of copulatory 

 organs and genital openings on the ventral 

 side. Leeches (Fig. 105) are abundant in 

 fresh water but also occur in salt water and 

 on land. Many of them are brilliantly col- 

 ored and bear elaborate color patterns. We 

 commonly think of leeches as bloodsuckers; 

 large numbers, however, are predaceous, 

 that is, they do not act as bloodsucking 

 parasites, but devour other small animals 

 such as earthworms and mollusks. They are 

 themselves preyed upon by birds such as the 

 bittern, reptiles, flatworms, and other ani- 

 mals. 



External annulation is not indicative of 

 the true number of segments; there may be 

 several external annulations for every seg- 

 ment as shown by internal organs (Fig. 

 106). 



The principal characteristics are exhibited 

 by Hirudo medicinalis, which is about 4 

 inches long but is capable of great contrac- 

 tion and elongation. The suckers are used 



as organs of attachment. Figure 106 illus- 

 trates the principal structures of a leech. The 

 digestive tract is fitted for digestion of the 

 blood of vertebrates, which forms the prin- 

 cipal food of some leeches. The mouth lies 

 in the anterior sucker and is provided with 

 three jaws armed with chitinous teeth for 

 biting. Blood is sucked up by the dilatation 

 of the muscular pharynx. The short esopha- 

 gus leads from the pharynx into the crop, 

 which has 1 1 pairs of lateral branches. Here 

 the blood is stored until digested in the 

 small globular stomach. Because of its enor- 

 mous crop, a leech is able to ingest three 

 times its own weight in blood; and, since it 

 may take as long as 9 months to digest this 

 amount, meals are few and far between. 



Respiration is carried on mainly through 

 the surface of the body. Waste products 

 are extracted from the blood and coelomic 

 fluid by 17 pairs of nephridia. Leeches are 

 hermaphroditic, but the eggs of one animal 

 are fertilized by sperms from another leech. 

 Copulation and formation of a cocoon are 

 similar to those processes in the earthworm. 

 Other leeches carry their eggs on the ventral 

 side, and some deposit them on stones. 



Metamerism 



The biological principle of body segmen- 

 tation is called metamerism. This is ex- 

 hibited in the true annelids and is here en- 

 countered for the first time. This type of 

 structure is of considerable interest since 

 the most successful groups in the animal 

 kingdom, the Arthropoda and Vertebrata, 

 have their parts metamerically arranged. 

 How this condition has been brought about 

 is still doubtful, but many theories have 

 been proposed to account for it. According 

 to one view, the body of a metameric ani- 

 mal has evolved from that of a non-seg- 

 mented animal by transverse fission. The 

 individuals thus produced remained united 

 end to end and gradually became integrated 

 both structurally and physiologically so that 

 their individualities were united into one 



